The interrelated effects of diet fat (lard, high oleic safflower oil, and high linoleic safflower oil); cholesterol, and phytosterols on cholesterol absorption and blood lipids will be studied. The effect of time of ingestion of phytosterols relative to meal feeding will be included in this study. Either Sprague-Dawley of BHE rats, whichever strain shows greater hypercholesterolemic response to diet cholesterol, will be used in this investigation. Studies are planned to further test the hypothesis that the diet of the developing animal can result in permanent changes in adaptive enzyme activities. Preliminary data are given which indicate such changes in fatty acid synthetase, saturated acyl desaturase and HMG-CoA reductase. The effect of aging on the usefulness of two strains of rats, Sprague-Dawley and BHE, as models for relating diet and serum cholesterol will be studied.